Polar S Series Speed Sensor (Please note this sensor is no longer avalible, it has been discontinued.  Polar says It can be fixed if you send it in)

The Polar Speed Sensor is compatible with the Polar S-520i, S-610, S-625X, S-710, S-720, and S-725 Heart Rate Monitors. The Polar Speed Sensor is a wireless sensor measuring cycling speed and distance via a magnet rotating on the front or the rear wheel depending on the sensor. The Polar Speed Sensor is shipped with a Polar Bike Mount.